With recent concerns about the exhaust of fossil energy and environmental pollution, interests in electric vehicles or hybrid vehicles using electrical energy instead of fossil energy are increasing.
The electric vehicles or hybrid vehicles use a high capacity battery pack, and the battery pack includes a plurality of cells capable of repeatedly charging and discharging. During charging/discharging of the battery pack, it requires a proper maintenance of the state of charge (SOC) of each cell and protection of the battery pack from abnormal circumstances such as over-charging or over-discharging. Thus, it needs to periodically measure and monitor the voltage of each cell using a cell voltage measuring apparatus.
FIG. 1 is a circuit diagram of a conventional battery cell voltage measuring apparatus 10.
Referring to FIG. 1, the conventional battery cell voltage measuring apparatus 10 comprises a floating capacitor (C), a first switch (SW1), a second switch (SW2), a cell voltage measuring circuit 20, an A/D converter 30 and a controller 40.
The first switch (SW1) is turned on by the controller 40 so as to make a cell voltage measurement. Accordingly, the voltage of each cell (B) is charged on each corresponding floating capacitor (C). After charging of the cell voltage, all the first switch (SW1) is turned off, so that the cell voltage is held on the floating capacitor (C).
After charging and holding of the cell voltage, the second switch (SW2) is turned on in sequence according to a preset order. Accordingly, the voltage (cell voltage) held on each floating capacitor (C) is applied to the cell voltage measuring circuit 20 in sequence.
The cell voltage measuring circuit 20 measures the voltage held on each floating capacitor (C) and applied thereto in sequence, and outputs an analog voltage signal corresponding to each cell voltage to the A/D converter 30. Then, the A/D converter 30 converts the analog voltage signal into a digital voltage signal of a predetermined bit and outputs the digital voltage signal to the controller 40.
The controller 40 controls the overall operation of the first switch (SW1) and the second switch (SW2), and receives a digital voltage signal of each cell (B) outputted from the A/D converter 30 and stores the digital voltage signal in a memory (not shown). And, the controller 40 controls the charge/discharge of each cell (B) using voltage data of each cell (B) stored in the memory, and performs various battery protection operations including prevention of over-charging or over-discharging.
The cell voltage measuring circuit 20 includes a differential amplifier for outputting a voltage signal corresponding to the voltage between both terminals of the floating capacitor (C) to the A/D converter 30. However, the conventional cell voltage measuring circuit 20 has cell voltage measuring lines L1 to L4 to measure the voltage of a plurality of cells (four cells) using a single differential amplifier.
As mentioned above, the conventional cell voltage measuring apparatus 10 has cell voltage measuring lines L1 to L4 to measure the voltage of four cells using a single differential amplifier. Thus, a polarity of the voltage between both terminals of the floating capacitor (C) should be inverted so as to measure the voltage of even-numbered cells. For this purpose, the cell voltage measuring circuit 20 has a polarity inversion circuit therein, which results in a complicated circuit structure of the cell voltage measuring apparatus 10. And, the voltage of a plurality of cells is measured using a single differential amplifier. Thus, high potential is applied to the differential amplifier when voltages of upper side cells are measured. For this reason, it is needed to use a high durability device capable of standing the high potential as a differential amplifier. However, the high durability device is expensive, and thus, it causes an increases in manufacturing cost of the cell voltage measuring apparatus.